1. Field of the Invention
This invention relates to a flexible multilayer packaging material showing no reduction in gas barrier properties even under extremely severe flexing fatigue conditions. More specifically, it provides a flexible multilayer packaging material comprising a thin layer composed of a saponified product of an ethylene-vinyl acetate copolymer (hereinafter referred to as the EVOH) having gas barrier properties against oxygen, carbon dioxide etc. as an intermediate layer, said flexible multilayer packaging material being further provided with linear low-density polyethylene layers as surface layers on both sides of said multilayer ppackaging material. This material retains excellent gas barrier properties against extremely severe flexing fatigue exerted on said packaging material on transport or handling of a tight container of an easily deteriorating product packaged with said packaging material and hence being effective in prevention of deterioration in quality of the product to be packaged.
2. Description of the Prior Art
The essential function of a flexible multilayer packaging material is preservation, that is, prevention of deterioration in quality of a product to be packaged, and therefore, in said packaging material, vibration endurance, flexing endurance etc. are especially required, and in particular, where it is used as an inner container for the so-called bag in box or bag-in-carton, which is "a container of a combination of (1) a foldable plastic thin-wall inner container and (2) an outer corrugated box or carton having stackability, portability and printability", a high degree of said characteristics is required. As said packaging material, various plastic films are used by lamination by making the best use of their own characteristics. For example, a combination of a base film for maintaining mechanical strength with a heat-sealable material is most common, and the material is selected according to the requirements of the product to be packaged. In particular, in a case where the gas barrier properties against oxygen etc. of the base film are inadequate, there is employed a method of providing the base layer with a barrier layer having an even higher degree of gas barrier properties thereon and laminating a thermoplastic resin layer of a heat-sealable material with said barrier layer as an intermediate layer so as to serve as at least one outer layer. For example, since the basic material for the inner container of the conventional bag-in-box or bag-in-carton has always heat-sealable sites, the material is mainly composed of a heat-sealable polyethylene, especially a low-density polyethylene. However, an ethylene-vinyl acetate copolymer resin is more preferably employed because (1) the basic material is required to have physical strength especially, vibration endurance and flexing endurance as described above in that it must be foldable, which is a special feature of the bag-in-box or bag-in-carton, and (2) since the content is a liquid, and so forth, the basic material should be of good stress crack resistance. Further, where gas barrier properties against oxygen etc. are required because of greater demand for superior properties, said inner containers of combinations of e.g. nylon films, saran-coated nylon films, aluminum-deposited nylon films, aluminum-deposited polyester films etc. have begun to be put to practical use. In order to impart excellent gas barrier properties, saponified products of ethylene-vinyl acetate copolymer, polyvinyl chloride, aluminum foil, deposited films of metal etc. and the like are employed. However, although they are excellent in gas barrier properties, their mechanical strength is lower and cannot stand flexing fatigue. Therefore, they are used by being laminated between a base layer having excellent mechanical strength and a heat-sealable material. Even so, where, for example, used as a constituting material for an inner container of a bag-in-box or bag-in-carton, since pinholes are generated in said constituting material or, even at a stage where such pinholes have not yet been generated, the barrier properties are lowered due to cracks, pinholes etc. generated in said barrier layer used as the intermediate layer, or for other reasons, it is impossible to retain the excellent gas barrier properties against extremely severe flexing fatigue. None of these compositions has hitherto been satisfactory for practical use. The behavior of multilayer packaging materials utilizing layers mainly comprising a polyvinylidene chloride resin, an aluminum foil, a deposited resin layer with a metal etc. and the like as a barrier layer are disclosed in, for example, Japanese Patent Application Laid-open No. 7477/1980. In other words, when said packaging material is actually used for packaging and the packaged goods are transported and handled, the gas barrier properties thereof are not always satisfactory. It is due to damage in said barrier layer positioned as the intermediate layer that the desired characteristics of practical storability after the primary distribution often turns out unreliable. As the material for the intermediate layer provided for improving the gas barrier properties, an EVOH resin is most excellent and preferably used as a barrier material for containers having various multilayer films or a multilayer structure. This is because EVOH resin has the extremely advantageous feature that it not only has especially excellent gas barrier properties but also has excellent transparency, oil resistance, printability, moldability etc. and hence does not interfere with the characteristics inherent in the base material. Nevertheless, in instances where flexing endurance is particularly required, there has hitherto been no example of an EVOH resin being satisfactorily used as a barrier layer for a multilayer packaging material. In particular, as described hereinabove, it is especially required for the packaging material to withstand flexing fatigue due to vibration in transport. There has hitherto been no EVOH resin which has been used as an inner container in a bag-in-box or bag-in-carton having gas barrier properties against oxygen etc. and satisfying the above-described requirements. Therefore, the development of a flexible multilayer packaging material utilizing an EVOH layer as a barrier layer and having flexing fatigue strength which can withstand vibration in transport as well as excellent barrier properties has been one of the essential problems to be solved.
Further, Japanese Utility Model Application Laid-open No. 23353/1972 describes a multilayer packaging material comprising an EVOH as an intermediate layer and a low-density polyethylene (branched-chain low-density polyethylene) as inner and outer layers, but such a multilayer packaging material is extremely poor in flexing endurance as demonstrated in Comparative Example 1 described hereinbelow.
Furthermore, Japanese Patent Application Laid-open No. 12008/1980 (corresponding U.S. Pat. No. 4,265,949) describes a packaging material of a laminate of a linear low-density polyethylene and an EVOH, but even with the packaging material of such a constitution, the flexing endurance is still unsatisfactory as demonstrated in Comparative Example 2 described hereinbelow.